This disclosure is based on applications No. H10-363664 filed in Japan on Dec. 22, 1998 and No. H11-005056 filed in Japan on Jan. 12, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a compact and inexpensive zoom lens system particularly suited for use in digital still cameras.
2. Description of the Prior Art
In recent years, as personal computers become more prevalent, digital still cameras that allow easy storage of image data on a recording medium such as a floppy disk have been coming into wider use. This trend has created an increasing demand for more inexpensive digital still cameras. This in turn has created an increasing demand for further cost reduction in imaging optical systems. On the other hand, photoelectric conversion devices have come to have an increasingly large number of pixels year by year, which accordingly demands imaging optical systems that offer higher and higher performance. To comply with such requirements, it is necessary to produce a high-performance imaging optical system at comparatively low cost.
To achieve this objective, for example, Japanese Laid-open Patent Applications Nos. H1-183615 and H9-311273 propose optical systems having a first lens unit of a negative-negative-positive configuration and a second lens unit of a positive-negative-positive configuration. Moreover, the optical systems proposed in Japanese Laid-open Patent Applications Nos. H7-113956, H6-300969, and H7-63991 have a second lens unit including a doublet lens element formed by cementing together negative lens elements; and the optical system proposed in Japanese Laid-open Patent Application No. H5-93858 has a second lens unit including a doublet lens element formed by cementing together, from the object side, a positive lens element and a negative lens element. If a doublet lens element is considered to be a single lens element, it is assumed that those optical systems are each composed of a first lens unit of a negative-negative-positive configuration and a second lens unit of a positive-negative-positive configuration.
Furthermore, Japanese Laid-open Patent Applications Nos. H6-201993 and H1-191820 propose optical systems that are composed of a first lens unit having a negative optical power, a second lens unit having a positive optical power, and a third lens unit having a positive optical power and employ a plastic lens element.
In the optical systems proposed in the above-mentioned patent applications, however, there is still plenty of room for improvement from the viewpoint of miniaturization, high performance, and cost reduction.
An object of the present invention is to provide a compact, high-resolution, and low-cost zoom lens system suitable, in particular, for use in a digital still camera by arranging plastic lens elements effectively in a two-unit zoom lens system of a negative-positive configuration.
To achieve the above object, according to one aspect of the present invention, a zoom lens system includes, from the object side, a first lens unit and a second lens unit. The first lens unit is composed of a negative, a negative, and a positive lens element and has a negative optical power as a whole. The second lens unit is composed of a positive, a negative, and a positive lens element and has a positive optical power as a whole. In the zoom lens system, zooming is achieved by varying the distance between the first and second lens units, and at least one of those lens elements is a plastic lens element.
According to another aspect of the present invention, a zoom lens system includes, from the object side, a first lens unit having a negative optical power and a second lens unit having a positive optical power. In the zoom lens system, zooming is achieved by varying the distance between the first and second lens units, and at least a negative lens element and a positive lens element of the lens elements included in the lens units are plastic lens elements that fulfill the following condition:
xe2x88x921.2 less than xcfx86Pi/xcfx86Wxc3x97hi less than 1.2 
where
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end;
xcfx86Pi represents the optical power of the ith plastic lens element; and
hi represents the height of incidence at which a paraxial ray enters the object-side surface of the ith plastic lens element at the telephoto end, assuming that the initial values of the converted inclination xcex11 and the height h1, for paraxial tracing, are 0 and 1, respectively.
According to another aspect of the present invention, an image taking apparatus is composed of a zoom lens system, a photoelectric conversion device, and an optical low-pass filter. The photoelectric conversion device has a light sensing surface on which an image is formed by the zoom lens system. The optical low-pass filter is disposed on the object side of the photoelectric conversion device. The zoom lens system is composed of, from the object side, a first lens unit and a second lens unit. The first lens unit is composed of a negative, a negative, and a positive lens element, and has a negative optical power as a whole. The second lens unit is composed of a positive, a negative, and a positive lens element, and has a positive optical power as a whole. In the zoom lens system, zooming is achieved by varying the distance between the first and second lens units, and at least one of those lens elements is a plastic lens element.
According to another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit has a negative optical power. The second lens unit is composed of at least a positive and a negative lens element, and has a positive optical power. The third lens unit has a positive optical power. In the zoom lens system, zooming is achieved by moving at least two lens units so as to vary the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the lens units is a plastic lens element that fulfills the following conditions:
xe2x88x920.8 less than Cpxc3x97(Nxe2x80x2xe2x88x92N)/xcfx86W less than 0.8 
xe2x88x920.45 less than M3/M2 less than 0.90(where xcfx86T/xcfx86W greater than 1.6) 
where
Cp represents the curvature of the plastic lens element;
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end;
Nxe2x80x2 represents the refractive index of the object-side medium of the aspherical surface for the d line;
N represents the refractive index of the image-side medium of the aspherical surface for the d line;
M3 represents the amount of movement of the third lens unit (the direction pointing to the object side is negative with respect to the wide-angle end);
M2 represents the amount of movement of the second lens unit (the direction pointing to the object side is negative with respect to the wide-angle end); and
xcfx86T represents the optical power of the entire zoom lens system at the telephoto end.
According to another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit is composed of at least a positive and a negative lens element, and has a negative optical power. The second and third lens units have a positive optical power. In the zoom lens system, zooming is achieved by moving at least two lens units so as to vary the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the first lens unit is a plastic lens element that fulfills the following conditions:
|xcfx86P/xcfx861| less than 1.20 
0.20 less than |xcfx861/xcfx86W| less than 0.70 
xe2x88x920.45 less than M3/M2 less than 0.90(where xcfx86T/xcfx86W greater than 1.6) 
where
xcfx86P represents the optical power of the plastic lens element;
xcfx861 represents the optical power of the first lens unit;
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end;
M3 represents the amount of movement of the third lens unit (the direction pointing to the object side is negative with respect to the wide-angle end);
M2 represents the amount of movement of the second lens unit (the direction pointing to the object side is negative with respect to the wide-angle end); and
xcfx86T represents the optical power of the entire zoom lens system at the telephoto end.
According to another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit has a negative optical power. The second lens unit is composed of at least a positive and a negative lens element, and has a positive optical power. The third lens unit has a positive optical power. In the zoom lens system, zooming is achieved by varying the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the second lens unit is a plastic lens element that fulfills the following conditions:
|xcfx86P/xcfx862| less than 2.5 
0.25 less than xcfx862/xcfx86W less than 0.75 
where
xcfx86P represents the optical power of the plastic lens element;
xcfx862 represents the optical power of the second lens unit; and
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end.
According to another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit has a negative optical power. The second and third lens units have a positive optical power. In the zoom lens system, zooming is achieved by moving at least two lens units so as to vary the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the third lens unit is a plastic lens element that fulfills the following conditions:
xe2x88x920.30 less than M3/M2 less than 0.90 
|xcfx86P/xcfx863| less than 1.70 
0.1 less than xcfx863/xcfx86W less than 0.60
where
M3 represents the amount of movement of the third lens unit (the direction pointing to the object side is negative with respect to the wide-angle end);
M2 represents the amount of movement of the second lens unit (the direction pointing to the object side is negative with respect to the wide-angle end);
xcfx86P represents the optical power of the plastic lens element;
xcfx863 represents the optical power of the third lens unit; and
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end.
According to another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit has a negative optical power. The second and third lens units have a positive optical power. In the zoom lens system, zooming is achieved by moving at least two lens units so as to vary the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the first lens unit and at least one of the lens elements included in the second lens unit are plastic lens elements that fulfill the following conditions:
xe2x88x921.4 less than xcfx86Pi/xcfx86Wxc3x97hi less than 1.4 
0.5 less than log(xcex22T/xcex22W)/log Z less than 2.2 
where
xcfx86Pi represents the optical power of the ith plastic lens element;
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end;
hi represents the height of incidence at which a paraxial ray enters the object-side surface of the ith plastic lens element at the telephoto end, assuming that the initial values of the converted inclination xcex11 and the height h1, for paraxial tracing, are 0 and 1, respectively;
xcex22W represents the lateral magnification of the second lens unit at the wide-angle end;
xcex22T represents the lateral magnification of the second lens unit at the telephoto end;
Z represents the zoom ratio; and
log represents a natural logarithm (since the condition defines a proportion, the base does not matter).
According to another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit has a negative optical power. The second lens unit is composed of at least a positive and a negative lens element, and has a positive optical power. The third lens unit has a positive optical power. In the zoom lens system, zooming is achieved by moving at least two lens units so as to vary the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the first lens unit and at least one of the lens elements included in the third lens unit are plastic lens elements that fulfill the following conditions:
xe2x88x921.4 less than xcfx86Pi/xcfx86Wxc3x97hi less than 1.4 
xe2x88x921.2 less than log(xcex23T/xcex23W)/log Z less than 0.5 
where
xcfx86Pi represents the optical power of the ith plastic lens element;
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end;
hi represents the height of incidence at which a paraxial ray enters the object-side surface of the ith plastic lens element at the telephoto end, assuming that the initial values of the converted inclination a 1 and the height h1, for paraxial tracing, are 0 and 1, respectively;
xcex23W represents the lateral magnification of the third lens unit at the wide-angle end;
xcex23T represents the lateral magnification of the third lens unit at the telephoto end;
Z represents the zoom ratio; and
log represents a natural logarithm (since the condition defines a proportion, the base does not matter).
According to still another aspect of the present invention, a zoom lens system is composed of, from the object side, a first lens unit, a second lens unit, and a third lens unit. The first lens unit has a negative optical power. The second lens unit is composed of at least a positive and a negative lens element, and has a positive optical power. The third lens unit has a positive optical power. In the zoom lens system, zooming is achieved by moving at least two lens units so as to vary the distance between the first and second lens units and the distance between the second and third lens units, and at least one of the lens elements included in the second lens unit and at least one of the lens elements included in the third lens unit are plastic lens elements that fulfill the following conditions:
xe2x88x921.4 less than xcfx86Pi/xcfx86Wxc3x97hi less than 1.4 
xe2x88x920.75 less than log(xcex23T/xcex23W)/log(xcex22T/xcex22W) less than 0.65 
where
xcfx86Pi represents the optical power of the ith plastic lens element;
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end;
hi represents the height of incidence at which a paraxial ray enters the object-side surface of the ith plastic lens element at the telephoto end, assuming that the initial values of the converted inclination xcex11 and the height h1, for paraxial tracing, are 0 and 1, respectively;
xcex22W represents the lateral magnification of the second lens unit at the wide-angle end;
xcex22T represents the lateral magnification of the second lens unit at the telephoto end;
xcex23W represents the lateral magnification of the third lens unit at the wide-angle end;
xcex23T represents the lateral magnification of the third lens unit at the telephoto end; and
log represents a natural logarithm (since the condition defines a proportion, the base does not matter).